Historically commercial harvesting of mollusks, e.g., oysters, clams, mussels, etc., has been largely dependent on naturally available cultivation terrain and conditions. Thus, the majority of oysters, clams, etc. are harvested from naturally existing areas in which the mollusks typically exist or are implanted. In other words, commercial harvesters of mollusks have, in large, simply harvested mollusks as they existed in nature or used naturally occurring propagation areas for seeding for subsequently harvesting.
Oysters, clams and similar shellfish undergo complex changes during their initial stages of growth. In order to grow an economically significant proportion of a crop of spat to marketable size, two things are of basic importance, i.e., availability of food and protection from predators.
A land-based hatchery/nursery provides protection, but must also provide food for the early, most vulnerable stages of the shellfish. As the juveniles grow, they require more food so a condition of economic impracticality is reached. In many natural situations, the algae utilized as food is abundant, but predators also are abundant. Enclosures may be used as protection against predators, but fouling organisms grow rapidly under such conditions so the enclosures must be cleaned at regular intervals to insure adequate water circulation through the enclosures.
As most mollusks are bottom dwelling creatures, conventional enclosures are bottom oriented, e.g., cages, and have been labor intensive as regards cleaning and sorting. Culture systems, such as oriental string culture for oysters, may be considered exceptions, but are nevertheless labor intensive.
It has long been recognized that the "natural" form of mollusk cultivation and harvesting is highly inefficient for many reasons, e.g., the attack of predators on the spat. Hence, considerable work has been done to try to render the cultivation and harvesting of mollusks a more efficient operation as shown by U.S. Pat. Nos.:
______________________________________ 2,922,393 3,811,411 3,017,856 3,909,971 3,017,857 4,328,764 3,678,900 4,434,743. 3,766,888 ______________________________________
This present invention provides new apparatus and methods that permit the cultivation and harvesting of mollusks to be much improved over prior methods and apparatus with the result that greatly improved quantities of marketable mollusks can be harvested for each unit of aquaculture area and unit amount of personnel time expended thereon as compared to what could be obtained prior to this invention.